I. Field of the Invention
This invention relates to a method and apparatus for sorting particle matter, such as wood chips used in the manufacture of paper pulp.
II. Prior Art and Other Considerations
In the manufacture of paper pulp from wood chips it is economically advantageous that the constituent wood chips have a relatively uniform thickness in a desired thickness range. For a given mass of chips the desired thickness range is dependent upon several factors, such as characteristics of the wood stock from which the chips were generated and the condition of the chips, e.g., whether a significant number of the chips are fissured. An appropriate critical thickness range for commercial softwood chips, for example, is from about 6 millimiters to 8 millimeters. Prior art research and literature relative to chip thickness are discussed by R. D. Christie in articles entitled "Chip Screenings For Pulping Uniformity," 1986 Pulping Conference, pp. 551-561 (1986), and "Chip Thickness and Its Effect On Pulping Needs Further Study," Pulp & Paper Canada 87.9 (1986).
It is expedient, therefore, that a mass of chips emanating from a chipper of the like be sorted so that only chips belonging to the desired thickness range are actually introduced into the paper pulp process. To this end, devices generally known as "disc screens" have been used to separate or sort wood chips of varying thicknesses. Disc screens typically comprise a plurality of rows of revolvable disc members with the discs in each row disposed in spaced parallel relation along a common horizontal axis of rotation. The discs of adjacent rows intermesh radially with equal axial spaces therebetween to permit chips of no greater thickness than such spaces to pass downwardly between the intermeshing discs. The width of the spaces are thus controlled to correspond with the maximum desired thickness of accepted chips.
One prior art mode of arranging the rows of discs in a disc screen is for the axes of rotation for all the rows to be aligned in a horizontal plane as in the manner shown in U.S. Pat. No. 4,452,694 to Christensen et al. In such an arrangement, infeed occurs above a first of the rows of discs. The infeed is undirectionally transported over the discs toward output device. Chips having thickness less than the width of the space between adjacent intermeshing discs drop through the spaces, while thicker chips are transported to the last of the rows of discs and are discharged to the output device.
Another prior art mode is to arrange the rows of discs in a disc screen in a trough-like configuration as shown in U.S. Pat. No. 4,377,474 to Lindberg. Infeed of chips occurs proximate the center of the trough. The discs are rotated so that chips are thrown upwardly and outwardly from the center of the trough. Chips of desired thickness fall through the spacing between the intermeshed discs; larger chips are discharged from an outlet end of the trough.
Regardless of the mode of arrangement of the rows of discs, prior art disc screens, when operated without auxiliary screens, are generally incapable of filtering out very fine or undesized particulate matter such as dirt, grit, and bark particles. This undesized particulate matter is detrimental to the pulping process.
In addition, such undersize particulate matter can have an abrasive effect when striking the discs employed in the disc screen. Since, in prior art disc screens, infeed is consistently introduced into the same portion of the disc screen, and since the undersized particulate matter immediately passes through the disc spacing in the introductory portion of the screen, introductory discs suffer inordinate wear and require frequent replacement.
Moreover, in prior art disc screens, the chips must be oriented to stand on edge above the space between the intermeshing discs in order to pass therethrough. Since this is an unnatural chip orientation, the chips must be agitated in hopes that they will momentarily acquire this on-edge orientation and fall through the spaces. Numerous structured and operational modifications have been proposed to facilitate desirable chip orientation, including varying disc rotational speeds; varying the slope for trough-like disc screens; and varying disc size and shape. Prior art modifications have met with only limited success, however, in achieving desired chip orientation.
In view of the foregoing, it is an object of the present invention to provide method and apparatus for effectively sorting particles of desired thickness.
An advantage of the present invention is the provision of method and apparatus wherein an incoming mass of chips is distributed over a large portion of a disc screen.
An advantage of the present invention is the provision of method and apparatus for a disc screen wherein essentially uniform wear of the discs reduces the frequency of disc replacement.
Another advantage of the present invention is the provision method and apparatus for facilitating a desired orientation of chips in a disc screen so that acceptably sized chips can readily fall in spaces between intermeshing discs.
A further advantage by the present invention is the provision of method and apparatus for facilitating the expulsion of overthick chips from a disc screen.
Yet another advantage of the present invention is the provision of a durable disc for utilization in a disc screen.